Attention has been drawn to a hydrogen-oxygen fuel cell as a power generating system which presents substantially no adverse effects on the global environment because in principle, its reaction product is water only. Polymer electrolyte fuel cells were once mounted on spaceships in the Gemini project and the Biosatellite project, but their power densities at the time were low. Later, more efficient alkaline fuel cells were developed and have dominated the fuel cell applications in space including space shuttles in current use.
Meanwhile, with the recent technological progress, attention has been drawn to polymer fuel cells again for the following two reasons: (1) Highly conductive membranes have been developed as polymer electrolytes and (2) it has been made possible to impart extremely high activity to the catalysts for use in gas diffusion electrodes by using carbon as the support and incorporating an ion exchange resin in the gas diffusion electrodes so as to be coated with the ion exchange resin.
However, a perfluoropolymer having sulfonic groups to be used as a polymer contained in a membrane and an electrode usually has unstable terminal groups with a C—H bond, such as —COOH groups, —CF═CF2 groups, —COF groups and —CF2H groups at some molecular chain terminals, and therefore, there was such a problem that a polymer gradually decomposes during long-term fuel cell operations, followed by decreasing the power generation voltage. In addition, there was such a problem that the fuel cell operation cannot be conducted because decrease of the mechanical strength due to the polymer decomposition, locally causes pinholes, breaking, abrasion or the like.
The above problems are caused by the presence of such unstable functional groups at some molecular chain terminals of a fluorine-containing polymer, and as methods for stabilizing such molecular chain terminals, for example, the following methods have been proposed.
A method of hydrothermal treatment of a tetrafluoroethylene/hexafluoropropylene copolymer (hereinafter referred to as a TFE/HFP copolymer) at a high temperature to convert —COOH groups into —CF2H groups (See Patent Document 1).
A method of decarboxination and fluorination of a fluorine-containing polyether having a low molecular weight by using fluorine gas in a liquid state or a state as dissolved in an inert solvent, to stabilize terminal groups (See Patent Document 2).
A method of shearing a TFE/HFP copolymer by a twin-screw extruder at a high temperature, followed by treating with fluorine gas (See Patent Document 3).
A method of treating a tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer (hereinafter referred to as a TFE/PFVE copolymer) by contacting it with fluorine gas in the form of pellets (See Patent Document 4).
A method of treating a TFE/PFVE copolymer by contacting it with fluorine gas in the form of granules (See Patent Document 5).
A method of treating a TFE/HFP copolymer or a TFE/PFVE copolymer by contacting it with fluorine gas in the form of a pulverized product having an average particle diameter of from 5 to 500 μm (See Patent Document 6).
A method of treating a TFE/PSVE copolymer by stirring a polymerization product obtained by solution polymerization or suspension polymerization in water, followed by contacting the resulting spherical granules having an average particle diameter of from 1 to 5 mm with fluorine gas (See Patent Document 7).
A method of subjecting a TFE/HFP copolymer or a TFE/PFVE copolymer to reactive heat treatment with oxygen and water by a kneader (See Patent Document 8).
A method of carrying out treatment of a TFE/HFP copolymer or a TFE/PFVE copolymer by melt-kneading in the presence of oxygen and melt-kneading in the presence of water in a single kneader (See Patent Document 9).
However, such methods are not designed for treatment of a polymer having ion exchange groups or their precursor groups, but designed for stability of a fluorine-containing polymer at the time of heat forming. Here, in this specification, precursor groups for ion exchange groups mean groups convertible into ion exchange groups by e.g. hydrolysis, and precursor groups for sulfonic groups may, for example, be —SO2F groups or —SO2Cl groups.
As a method of improving the stability of a fluorine-containing polymer containing ion exchange groups or their precursor groups, a treating method has been proposed wherein a perfluoropolymer having sulfonic groups is put in a shaking tube coated with nickel or a stainless steel container and contacted with fluorine gas (See Patent Document 10).
It is possible to reduce unstable terminals by means of such treatment with fluorine gas, whereby it is effective to improve the durability of the polymer. However, such treatment may sometimes be insufficient by itself depending upon the purpose.
Patent Document 1: U.S. Pat. No. 3,085,083 (Claim 1 and lines 24 to 66 in column 2)
Patent Document 2: U.S. Pat. No. 3,242,218 (Claim 1)
Patent Document 3: U.S. Pat. No. 4,626,587 (Claims 1 to 3)
Patent Document 4: JP-B-4-83 (Line 20 on page 4 to line 14 on page 5)
Patent document 5: JP-B-7-30134 (Claim 1)
Patent Document 6: JP-B-7-5743 (Claims 1 to 3)
Patent Document 7: JP-A-10-87746 (Claim 1)
Patent Document 8: JP-A-2000-198813 (Claim 1)
Patent Document 9: JP-A-2002-249585 (Claims 1 to 2)
Patent Document 10: JP-B-46-23245 (Claim 1)